1. Field of the Invention
The present invention relates generally to a capacitive device and more particularly to a capacitive device comprising an electrical interconnect useful for electric double layer capacitors and/or for capacitive deionization.
2. Technical Background
Capacitors, like batteries, store energy in the electrical field between a pair of oppositely charged conductive plates. Developed more than 250 years ago, capacitors are frequently used in electrical circuits as energy storage devices. In recent years, new families of capacitive devices have been developed which are based on charge separation of ions in solution and the formation of electrical double layers.
An electric double layer capacitor (EDLC) is an example of a capacitor that typically contains porous carbon electrodes (separated via a porous separator), current collectors and an electrolyte solution. When electric potential is applied to an EDLC cell, ionic current flows due to the attraction of anions to the positive electrode and cations to the negative electrode. Electric charge is stored in the electric double layer (EDL) formed along the interface between each polarized electrode and the electrolyte solution.
EDLC designs vary depending on application and can include, for example, standard jelly roll designs, prismatic designs, honeycomb designs, hybrid designs or other designs known in the art. The energy density and the specific power of an EDLC can be affected by the properties thereof, including the electrode and the electrolyte utilized. With respect to the electrode, high surface area carbons, carbon nanotubes, activated carbon and other forms of carbon and composites have been utilized in manufacturing such devices. Of these, carbon based electrodes are used in commercially available devices.
Capacitive Deionization (CDI) is a promising deionization technology, for instance, for the purification of water. In this context, positively and negatively charged electrodes are used to attract ions from a stream or bath of fluid. The ions form electric double layers on the surfaces of the electrodes, which are fabricated from some form of high surface area material, for example, a form of activated carbon. After interaction with the electrodes during the charging period, the fluid contains a lower overall ion content and is discharged. A volume of purge fluid is then introduced to the electrodes. The electrodes are then electrically discharged, thus releasing the trapped ions into the purge fluid. The purge fluid is then diverted into a waste stream and the process repeated.
Electrically connecting electrodes to a power source is a challenging aspect for EDLC and CDI applications. Typically, electrodes are delicate, thus mechanical stressing and straining of the electrodes should be minimized. Minimizing the deformations applied to the electrodes is difficult, especially while attempting to maximize the electrical and mechanical integrity of an electrical interconnect to the electrodes.
U.S. Pat. No. 5,954,937 relates to an interconnection for resorcinol/formaldehyde carbon aerogel/carbon paper sheet electrodes. The fluid flow path is located between the surfaces of the electrode sheets. The active surfaces of these electrode sheets are delicate and should be protected from mechanical stressing. The electrode sheets are bonded to a current collector, in this case, a titanium sheet using a conductive carbon filled adhesive. The large area of contact between the electrode sheet and the current collector insure relatively low overall resistance despite the moderately high resistivity of the adhesive interface.
U.S. Pat. No. 6,778,378 relates to electrodes which may be rolled from carbon and fibrillated polytetrafluoroethylene (PTFE). Electrodes formed in this fashion are thin flexible sheets which can be contacted by high normal compressive forces. Electrodes may be stacked up with sheets of current collector material and a separator material and then clamped with a compressive force to obtain good electrical contact. By controlling which electrodes and current collectors are in physical contact, a capacitive cell may be formed.
In commonly owned U.S. Pat. No. 6,214,204, monolithic, low back pressure porous electrodes are made by one of several methods, which include honeycomb extrusion, casting or molding from a phenolic resin-based batch. After curing, these parts are carbonized and activated to create high surface area carbon monoliths with good electrical conductivity.
Discs are made and assembled in a stack and spaced such that the discs are electrically isolated from each other. The discs are connected to anode and cathode current collector/bus bar assemblies utilizing wires.
A variety of other approaches to electrically interconnecting electrodes have been considered in the art with one or more disadvantages as described below. Brazing or soldering alloys typically will not withstand either the EDLC or the CDI electrochemical environments. Brazing and/or soldering to carbon is difficult due, in part, to the low strength of activated carbon. Conductive adhesives formulated using highly conductive metal powders are costly and/or are prone to corrosion. Conductive adhesives formulated using carbon powders generally have insufficient electrical conductivity for use in a capacitor.
Conductive wire or strip leads mechanically fastened around the perimeter of a capacitive device provide adequate performance for small electrodes. However the resistive losses introduced by conducting charge around the circumference of the electrode in a small diameter wire or thin strip lead degrade performance, and no simple means has been found to use this attachment scheme while incorporating a high efficiency current collector. Also, the logistics of attaching leads to individual electrodes are not appealing.
It would be advantageous to have a capacitive device comprising an electrical interconnect to a linear stack of electrodes, which does not jeopardize the mechanical integrity of the electrodes. Also, it would be advantageous to have the electrical interconnect be electrochemically inert. Further, it would be advantageous to develop a capacitive device, comprising interconnected monolithic high surface area carbon electrodes, which is capable of non-impeded fluid flow through the electrodes, which is useful for, for example, CDI.